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How Do You Know the Right Fuse for Your Circuit Protection Needs

BY: Christina Hansen

Circuit protection has become more complex than it once was. Today's market is filled with such an array of fuses there needs to be more detail and knowledge then there used to be for proper selection. Thanks to sophisticated power distribution systems, there now needs to be information providing more knowledge of fuse types, construction, and operation characteristics. It's important to stay informed on the latest developments as new types, constructions, characteristics, and classes are established.

The following are standards which are accepted and used as requirements by which manufacturers must design their fuses. These standards are continuously being updated, it's important to refer to the latest version when designing or constructing your next project.

The following information is provided to analyze several of these fuse classes, along with pertinent performance characteristics and ratings.

Class CC fuses.These non-renewable fuses are current-limiting and intended for the protection of components sensitive to short-time overloads, non-inductive loads, and short-circuit protection of motor circuits.

Current ratings range from 0A to 30A, and the voltage rating is 600VAC. Class CC fuses are available with DC ratings. The interrupting rating is 200kA rms symmetrical.

Class CC fuses must be labeled "Current Limiting," and may be labeled "Time Delay." The optional time-delay test requirements for this class of fuses (a minimum 12-sec opening time at 200% fuse current rating) is different than that of other larger body fuses.

Class G fuses.These fuses were specifically created for use in lighting and appliance panel boards that are equipped with a special fusible-switch unit. These are non-renewable cartridge fuses that are for use only in AC circuits where interrupting ratings to 100kA rms symmetrical are required. These can also be available with DC ratings.

G Class fuses conform to standard 248-5, and are rated for 600VAC (0A to 20A) and 480VAC (25A to 60A). They are available in four ferrule sizes: 0A to 15A, 16A to 20A, 1A to 30A, and 31A to 60A. The innovative design of this class of fuse was selected so as to prevent the interchangeability with any other fuse class.

Class G fuses are current-limiting and are able to be so labeled per UL 248-5, which prescribes the maximum peak let-through and I2t let-through values permitted for this class of fuse.

The time delay test feature is optional. It is important to realize that the test calls for a minimum opening time of 12 sec at 200% of the fuse ampere rating which differs Class H, K, and R fuses.

Class H fuses.This is a cartridge fuse and is intended for general purpose branch circuit, lighting circuit, and the protection of non-inductive equipment like electric ovens and resistance heaters. You can select from renewable and non-renewable Class H fuses. The renewable fuses allow the user to replace the internal fusible link after the fuse operates.

All Class H units are tested for short-circuit requirements and are available with DC ratings. The short-circuit power factors are relatively high: 0.45 to 0.50 for fuses rated 110A to 600A, and 0.85 to 0.90 for fuses rated 100A and less. The actual short-circuit power factors encountered in typical installations are on the order of the above test values, when the available short-circuit current is 10kA rms symmetrical or less.

With respect to the time delay feature of fuses, the renewable types cannot perform the time delay but the non-renewable can.

Class H fuses, both non-renewable and renewable, are often misapplied in the electrical industry for a couple reasons. First, their minimal 10kA interrupting rating is easy to exceed in an industrial plant or commercial building. Second, workers unfamiliar with the differences between devices might be tempted to install more than one link in a renewable fuse.

Class J fuses.These non-renewable fuses are current-limiting and conform to 248-8. Current ratings of UL Class J fuses range from 0A to 600A, and the voltage rating is 600VAC. Class J fuses are available with DC ratings. The interrupting rating is 200kA rms symmetrical. Fuses with 300kA interrupting ratings are available. Time-delay labeling is available if the fuse meets the UL optional time-delay test requirements of a minimum 10-sec opening time at 500% of fuse current rating.

Class K fuses.These non-renewable fuses are available in 250VAC and 600VAC ratings, with current ratings from 0A to 600A. Class K fuses are available with DC ratings.

The interrupting ratings may be 50kA, 100kA, or 200kA rms symmetrical. Class K-1 fuses provide the best degree of current limitation. On the other hand, Class K-5 fuses provide a lesser degree of current limitation.

The standards requirements for Class K fuses prescribe maximum peak let-through current and maximum I2t let-through energy for each class and all are current-limiting fuses. Interestingly, these fuses cannot be labeled as current limiting because they are interchangeable with Class H fuses, which are noncurrent limiting.

Class L fusesThese non-renewable fuses are current-limiting and conform to 248-10. They're designed for the protection of feeders and service entrance equipment.

Current ratings for UL Class L fuses range from 601A to 6000A, and the voltage rating is 600VAC. Class L fuses are available with DC ratings. Available case sizes are 800A, 1,200A, 1,600A, 2,000A, 2,500A, 3,000A, 4,000A, 5,000A, and 6,000A. The interrupting rating is 200kA rms symmetrical. (300kA fuses are available.)

As with the other classes of fuses conforming to 248 (G, J, R, and T), Class L fuses must be labeled "Current Limiting." They may be labeled "Time Delay," although the standard doesn't have a requirement for their time-delay characteristics.

Class R fusesThese non-renewable fuses are made in 250VAC and 600VAC ratings, with current ratings from 0A to 600A. Class R fuses are available with DC ratings. They have an interrupting rating of 200kA rms symmetrical and are separated into two separate classes, RK1 and RK5. RK1 fuses are available with interrupting ratings of 300kA.

Class R fuses must be labeled "Current Limiting" and may be labeled "Time Delay," which means that they meet the optional time-delay test by not opening in less than 10 seconds at 500% of fuse ampere rating.